The present invention relates to a process for purifying sulfur. More particularly, the invention is concerned with a novel technique for treating sulfur, having a high content of undesirable solid impurities, to remove such impurities and provide a purified sulfur product. Specifically, this invention relates to a process for purifying sulfur by means of centrifugation.
Sulfuric acid is the largest commodity chemical in the world in terms of tonnage, and the great majority of it is produced by the combustion of sulfur in sulfuric acid plants. Historically, mined natural sulfur has been the primary source of commercial sulfur, but this position has given way in recent years to sulfur recovered from crude oil and natural gas processing. Natural sulfur is mined primarily by the Frasch hot water injection process, with much lesser amounts obtained by traditional mining techniques. Recovered sulfur is obtained largely by the treatment of gaseous hydrogen sulfide streams in a Claus Process plant. Regardless of the source of the sulfur, all sulfuric acid plants require clean sulfur to feed to their burners in order to avoid burner fouling. Natural sulfurs contain small amounts of solid impurities from the mining processes. Recovered sulfurs usually contain catalyst fines and, if formed into solid prills, they also contain solid impurities resulting from storage and transfer operations. These solid impurities must be removed prior to combustion in the sulfuric acid plants. Solid prilled sulfur commands a substantial portion of the world sulfur market. A drawback of solid sulfur is its tendency to form sulfuric acid on the surface of the sulfur prills due to natural bacterial action. The sulfuric acid is usually neutralized by the addition of lime to the sulfur. This treatment results in suspending small gypsum particles throughout the molten sulfur, and these particles must also be removed prior to combustion.
Analysis of sulfur for solids involves burning the sulfur to obtain a solid residue, or ash. As a result, the solids impurities in sulfur are often referred to as the xe2x80x9cash contentxe2x80x9d or, simply, xe2x80x9cashxe2x80x9d. These solids impurities have a particle size that normally ranges from about 5 to about 100 microns, and are present in sulfur in amounts ranging anywhere from about 10 ppm to as much as 2,000 ppm and higher. The nature, particle size and amount of ash present vary with the source and the history of the sulfur. Salt crystals (sodium or potassium chloride), which comprise a significant portion of run-of-mine Frasch sulfur ash, normally originate from the deposit from which the sulfur is mined, and their presence makes the sulfur unacceptable for many purposes. Calcium sulfate and calcium carbonate are also sometimes present in Frasch sulfur. Components of recovered sulfur ash include calcium sulfate and calcium oxide. In addition, iron sulfides and other iron compounds are often found in both recovered sulfur ash and Frasch sulfur ash as a result of the sulfur coming into contact with iron pipes, tanks and the like. Depending on the type of storage facilities used, both recovered and Frasch sulfur may also contain solids like silica, silicates and other such soil components.
The removal of ash from sulfur for the purpose of improving sulfur purity has been the subject of extensive research, as well as commercial investigations and studies, some more successful than others. Cycloning, washing, adsorption and filtration have been tried by various sulfur producers with varying degrees of commercial success. Since sulfur solidifies at around 240xc2x0 F., the challenge of successfully purifying it is further burdened by the fact that the purification process usually must be carried out while the sulfur is molten. The difficulties in handling molten sulfur are illustrated in U.S. Pat. Nos. 4,149,836, 4,218,411, 4,944,769 and 5,071,332, which describe atomizers and granulators for converting molten sulfur into particulate sulfur and granulated sulfur, respectively. These patents, however, do not address the purification of sulfur. U.S. Pat. Nos. 2,941,868 and 3,042,503 address the purification of sulfur in its molten state by means of further heating and solvent extraction so as to produce a sulfur product of reduced carbon content. These techniques do not address, however, the removal of salt, ash or other such similar solid impurities from the sulfur. U.S. Pat. No. 3,474,911 describes the use of filtration to remove solid impurities from sulfur and provides a solution to one of the shortcomings associated with the use of such filtration systems. In this case, a special filter configuration is needed to detect filter failures and minimize contamination of the filtered product that would occur otherwise. Another shortcoming of sulfur filtration systems is that the sulfur has to be transported to a remote location from where it is produced in order to process it in a separate filtration plant, thus adding transportation and other related expenses to the cost of production.
It is an object of the present invention to provide a process for purifying sulfur with respect to solid impurities.
Another object of this invention is to provide a commercially practicable method and system for treating sulfur in its molten state so as to remove salt, ash and/or similar solid impurities.
An object of the present invention is to provide a practicable technique for purifying sulfur contaminated with solid impurities that is relatively low in equipment cost, as well as in operating and maintenance costs, and which results in insignificant sulfur losses.
A specific object of this invention is to provide a method for purifying sulfur with respect to solid impurities which is not burdened with the shortcomings attendant to filtration and other prior art methods of sulfur purification.
A further object of the present invention is to provide a system for purifying sulfur at the point or location where the sulfur is mined or originally recovered, thereby avoiding or minimizing the costs associated with transportation to and handling of the sulfur in special purification plants.
These and other objects of the invention will be apparent to those skilled in the art from the description that follows.
The process of the instant invention centers around the innovative use of centrifugation and, in particular, high centrifugal force (xe2x80x9cG forcexe2x80x9d) centrifugation under controlled conditions to effectively remove the salt and other components of the ash normally found in commercial sulfurs. The high G force centrifugation is best carried out in a xe2x80x9cdesludger centrifugexe2x80x9d at a G force (centrifugal force per unit mass) of at least 4,000. (A G force of 1 is the force exerted on a unit mass by the standard gravitational pull of the earth, where the standard acceleration of gravity is 9.80665 meters per second per second, or 32.174 feet per second per second.) Preferably, a G force of between about 4,000 and 12,000 is used.
Centrifugation in this fashion generates a product stream of purified sulfur and a waste stream of high-solids sulfur. A desludger centrifuge is a type of solid bowl disc centrifuge provided with a plurality of conical discs to effect centrifugal separation of solids from liquids at high G forces. An example of a desludger centrifuge is the clarifier centrifuge manufactured by Westfalia Separator AG, of Oelde, Germany, as Model SB 60. A simplified schematic description of a Model SB 60 clarifier centrifuge is shown in FIG. 1. Means for providing heat to the desludger centrifuge so as to maintain the temperature of the sulfur above 250xc2x0 F. are used to maintain the sulfur in molten state.
To be commercially successful the centrifuge should be able to concentrate the solid impurities in the molten sulfur to a high degree. The solids level in the resulting waste sulfur should be at least 1% by weight (10,000 parts per million by weight, or xe2x80x9cppmxe2x80x9d), and preferably at least 5% by weight (50,000 ppm), to minimize sulfur loses. The centrifuge also should be able to routinely discharge the high-solids waste sulfur automatically. High-solids sulfur has very high viscosity. Consequently, increased concentrations of high solids in sulfur would be expected to significantly limit the degree of separation achieved in the centrifuge and to negatively impact the ability of the centrifuge to automatically discharge the waste sulfur. Surprisingly, the process of this invention is able to concentrate solid impurities in the waste sulfur to as high as 35% by weight (350,000 ppm) and still achieve efficient high-solids separation and waste sulfur discharge. The reason for the low-viscosity like behavior of the high-solids sulfur during the high G force centrifugation of this invention is not clearly understood.
If large volumes of sulfur are to be treated, it has been found that a pretreatment step, where the molten sulfur is first subjected to high G force centrifugation in a nozzle bowl centrifuge, yields excellent results. A nozzle bowl centrifuge is a type of solid bowl disc centrifuge provided with a plurality of conical discs to effect centrifugal separation of solids from liquids at high G forces. It is similar to a desludger centrifuge in many respects, but the nozzle bowl centrifuge has multiple nozzles located at the bowl periphery which are used to continuously discharge high-solids waste sulfur. An example of a nozzle bowl centrifuge is the nozzle bowl centrifuge manufactured by Westfalia Separator AG, of Oelde, Germany, as Model SDA 130. The nozzle bowl centrifuge preferably operates at the same, or nearly the same, G-force range described above for the desludger centrifuge. The product sulfur stream from the nozzle bowl centrifuge is comparable in low-ash quality to that obtained from the desludger centrifuge, but the waste underflow stream from the nozzle bowl centrifuge is usually too low in solids content to allow for economical disposal. Instead, it has been found that the waste sulfur stream from the nozzle bowl centrifuge should be further processed, e, g., in a desludger centrifuge, as described above, to produce a second purified sulfur stream and a high-solids waste sulfur that is economical to dispose.
A commercial sulfur purification facility using the process of this invention may consist of one or more nozzle bowl centrifuges and desludger centrifuges, connected in series or in parallel, depending on the required sulfur throughput and the process capacities of the centrifuge(s). The raw sulfur may be fed to the nozzle bowl centrifuge(s), and the waste sulfur stream from the nozzle bowl centrifuge(s) may be further processed in a bank of parallel desludger centrifuges. Alternatively, it may be desirable to forego the nozzle bowl centrifuge(s) and effect the purification process with one or more desludger centrifuges operating in parallel.
Another aspect of the sulfur purification process is that the nozzle bowl centrifuge underflow waste sulfur stream may be further processed in a gravity settler, or a cyclone-gravity settler arrangement, to obtain a high-solids waste sulfur stream and a moderate-solids sulfur stream which then may be mixed with the low-solids product sulfur stream from the nozzle bowl centrifuge, or recycled to the feed of the nozzle bowl centrifuge. This process is generally inferior in solids removal to that achieved by a desludger centrifuge, but may suffice for some scenarios. In this process arrangement the waste sulfur underflow from the nozzle bowl centrifuge is preferably first fed to a cyclone in order to increase the solids density of the underflow and thereby reduce the required size of the gravity settling equipment. The moderate-solids overflow product sulfur stream from the cyclone is returned to the nozzle bowl centrifuge, or it may be mixed with the product sulfur stream from the nozzle bowl centrifuge. The underflow high-solids waste sulfur stream from the cyclone is sent to a gravity settler. The moderate-solids overflow product sulfur stream from the gravity settler is returned to the nozzle bowl centrifuge or blended with the low-solids sulfur stream from the nozzle bowl centrifuge. If the waste sulfur stream from the nozzle bowl centrifuge is very small, the cyclone may be omitted from the process.